function Queue() {
  this.items = []
  Queue.prototype.enqueue = (element) => {
    this.items.push(element)
  }
  Queue.prototype.dequeue = () => {
    return this.items.shift()
  }
  Queue.prototype.front = () => {
    return this.items[0]
  }
  Queue.prototype.size = () => {
    return this.items.length
  }
  Queue.prototype.isEmpty = () => {
    return this.items.length == 0
  }
  Queue.prototype.toString = () => {
    let resultString = ''
    for (let i = 0; i < this.items.length; i++) {
      resultString += this.items[i] + ' '
    }
    return resultString
  }
}

let queue = new Queue()
queue.enqueue(1)
queue.enqueue(2)
queue.enqueue(3)
queue.enqueue(4)
queue.enqueue(5)
queue.enqueue(6)
queue.dequeue()
queue.front()
queue.isEmpty()
queue.size()
queue.toString()